The present invention relates to microwave assisted chemistry, and in particular relates to a reaction vessel structure that can both withstand and release high pressures without catastrophic failure.
Microwave assisted chemistry refers to the use of microwaves to initiate or accelerate chemical reactions. Microwave assisted chemistry is particularly useful in adding energy to materials that are responsive to microwave radiation because under most circumstances, the resulting effects take place much more rapidly than they would if the reactions were initiated or accelerated using more conventional techniques such as convection or conduction heating.
As well understood by those familiar with the electromagnetic spectrum, the term “microwave” is often used generically to refer to radiation with wavelengths of between about 1000 and 500,000 microns (μ), and corresponding frequencies of between about 1×109 and 5×1011 Hertz (Hz). These are arbitrary boundaries, however, and other sources refer to microwaves as having frequencies of between about 108 Hz and 1012 Hz and wavelengths of between about 300 centimeters (cm) and 0.3 millimeters (mm). For commercial and consumer purposes in the United States, the available microwave frequencies are regulated by the Federal Communications Commission and are generally limited to certain frequencies such as 2450 megahertz (MHz). Because of the relatively long wavelength of microwave radiation, microwave assisted chemistry techniques are often carried out in closed vessels which are in turn placed inside a device that bears a superficial relation to a consumer microwave oven, but that is much more sophisticated in its source, waveguide, cavity, and control elements.
Microwave assisted chemistry can be used in a variety of chemical processes including moisture determination, ashing, digestion, extraction, and synthesis. Under some circumstances, these various techniques are preferably or necessarily carried out in closed vessels which, because of the generation or expansion of gases inside, must be able to withstand pressures significantly above atmospheric pressure.
Accordingly, a number of pressure vessels have been developed for high-pressure microwave assisted chemistry. Such vessels are typically formed of microwave transparent materials that offer the structural capabilities required to withstand such high pressures. High-strength polymers are exemplary of such materials and offer the required microwave transparency and resistance to chemical attack. Such materials tend to be brittle, however, so that failure under pressure tends to destroy the vessel quickly and release its contents suddenly. In order to complement these polymers and avoid catastrophic failure, the vessel or certain of its component parts typically include one or more composite materials, a common version of which includes textile materials such as fibers, yarns or fabrics.
Versions of such composite fabric vessels are disclosed, for example, in U.S. Pat. Nos. 5,427,741; 5,520,886 and 6,136,276, and published U.S. applications Nos. 20010022949 and 20020061372, all of which are commonly assigned with the present invention.
The composite sleeve structures have provided the opportunity to greatly increase the reaction pressures at which microwave assisted chemistry can be carried out, while avoiding some of the disadvantages of earlier generations of reaction vessels. In particular, the enhanced performance and controlled, non-shattering failure characteristics of composite vessels have permitted microwave assisted chemistry to be carried out at pressures as high as 800 pounds per square inch (psi) in the reaction vessel. As set forth in the cited patents, higher pressures can be accommodated to a certain extent by surrounding the reaction vessel with both the composite sleeve and a frame which holds the vessel in place and which urges the vessel lid or cap tightly against the reaction vessel.
There are, however, a number of reactions that can be carried out under elevated, but more moderate pressures. In particular, for carrying out certain types of reactions at temperatures of between about 200 and 250° C., a vessel should be able to withstand pressures of about 250 pounds per square inch (psi). Furthermore, many reactions can continue to take place successfully if the excess pressure can be temporarily relieved.
Vessels that safely release higher pressures are generally well understood, e.g. commonly assigned U.S. Pat. No. 5,230,865, but many provide for a one-time failure, i.e. the vessel releases pressure safely, but at the cost of the vessel or a component part. Furthermore, the pressure release tends to be total rather than controlled.
As another consideration, vessels for high-pressure microwave-assisted chemistry techniques have tended to be somewhat bulky for reasons that include the pressure requirements set forth above. As a complimenting feature, the microwave instruments used to heat reactions in these vessels generally need to be of a manageable size (footprint) in order to fit into expected areas of a laboratory. As a result, the maximum number of vessels that can be heated at one time in a typical microwave instrument tends to be between 12 and 16. In many common instruments, such as those illustrated in U.S. Pat. No. 5,230,865, the pressure-resistant vessels are arranged on a turntable so that they can be rotated through the cavity as the microwaves are applied. Such rotation helps provide an even amount of radiation to each vessel in accordance with the behavior of microwaves in such cavities.
Of course, the respective sizes of the vessels and microwave cavities fixes the number of reactions that can be carried out at any one time. This is becoming more limiting as chemistry has moved, with the aid of increasingly available and affordable computational power, to reaction schemes in which larger numbers of very similar, but not identical, reactions are carried out concurrently rather than consecutively. Accordingly, a need exists for smaller vessels that can be used in larger numbers in typical microwave instruments, but which still can withstand the required pressures, and which still can offer the desired capabilities for temporary, partial pressure release followed by resealing without permanent vessel damage or distortion.
As another problem, vessels of this type typically must be machine tightened (i.e. with a wrench-like torque force) in order to withstand the expected pressures. A need thus also exists for vessels that can be hand-tightened—thus eliminating extra steps, tools and time—while still providing the desired pressure-resistance and release capacities.